Double-feed antennas operating with waves of two frequencies of the same polarization



EARCH R001 Oct. 25, 1966 P. w. HANNAN 3, 8

DOUBLE-FEED ANTENNAS OPERATING WITH WAVES OF TWO FREQUENCIES OF THE SAME POLARIZATION Filed March 7, 1962 2 Sheets-Sheet 1 f HORIZ. POLAR. '-f VERT. POLAR. -f HORIZ.POLAR.

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P. w. HANNAN 3,281,850 ENNAS ES OF TWO OPERATING WITH WAV THE SAME POLARIZAT ION 2 Sheets-Sheet 2 ES OF Oct. 25, 1966 DOUBLE-FEED ANT NCI FREQUE Filed March 7, 1962 Tl i? WJf/ZZ M d L M FIG. 7b

FIG. 70.

FIG. 8b

FIG. 8a

DOUBLE-FEED ANTENNAS OPERATING WITH WAVES OF TWO FREQUENCIES OF THE SAME POLARIZATION Peter W. Hannan, Northport, N.Y., assignor to Hazeltine Research Inc., a corporation of Illinois Filed Mar. 7, 1962, Ser. No. 178,102 17 Claims. (Cl. 343-756) This invention relates to antennas required to operate independently on two separate frequencies of the same polarization and more specifically to such antennas which include one main reflector and a separate feed for waves of each such frequency. These antennas may also include one or more subreflectors.

To avoid possible confusion, the following usage should be kept in mind. In the specification and claims when an antenna is described as operating on two frequencies of the same polarization, this refers to the antenna as a whole and not to individual feeds of the antenna. The individual feeds may or may not operate with the same polarization as the complete antenna. This is possible since, while the complete antenna couples waves to and from free-space, the feeds cooperate with other components of the antenna such as a main reflector and subrefl-ectors which have frequency-selective polarization changing properties in accordance with the invention, so that polarizations may be altered prior to coupling to a feed.

Some antennas are required to operate independently with two separate frequencies. In the most common type of microwave antenna, the reflector type, this might be done by designing a special frequency-selective feeding system. However, such a technique does not usually permit an antenna to achieve its best performance at both frequencies because the feed system is common to both frequencies. Another technique might employ two feeds, one cross-polarized to the other. However, a simple antenna would then radiate two different polarizations at the two frequencies and this would be undesirable in some applications. The present invention relates to antennas which avoid these problems by utilizing frequency-selective reflector systems which allow two separate feeds to be employed, one for each frequency, while permitting the antenna to radiate the same polarization at both frequencies. The present invention is applicable to simple antennas using one or more feeds cooperating with a large reflector as well as to more complex types of multiple reflector antennas, such as Cassegrain antennas, which are presently coming into use.

The objects of this invention are to provide new and improved antennas, able to operate with waves of two frequencies of the same polarization, which avoid one or more disadvantages of prior art antennas; and to provide frequency-selective devices for use in such antennas.

In accordance with the invention, a double-feed antenna operating with two free-space polarized waves of the same polarization but of different frequencies comprises a main reflector, first feed means for first polarized waves of a first frequency, second feed means for second polarized waves of a second frequency, and frequency-selective means cooperating with the main reflector and the feeds for changing the polarization of waves of the second frequency.

Further in accordance with the invention, a frequencyselective transreflector, for transmitting waves of a first frequency and polarization and reflecting waves of a second lower frequency and the same polarization, comprises: a plurality of array mean-s for reflecting portions of incident waves of both said frequencies and transmitting the remaining portions of the waves; and spacing means attached to the array means for supporting them in spaced parallel relation with a spacing such that there is a sep- United States Patent 3,281,850 Patented Oct. 25, 1966 aration between array means which approximates a multiple of one-half of the effective wavelength at the first frequency and an odd multiple of one-quarter of the effective wavelength at the second lower frequency; whereby, the complete transreflector provides substantially the function of a half-wave resonator for waves of the first frequency and at the second lower frequency the reflections from the array means reinforce so that waves at the lower frequency are substantially completely reflected.

For a better understanding of the present invention, together with other and further objects thereof, reference is bad to the following description taken in connection with the accompanying drawings, and its scope will be pointed Antennas of FIGURES 1, 2 and 3 In FIGURES 1, 2 and 3, are shown three different types of antennas each of which includes two feeds and operate with free space waves of two frequencies of the same polarization. These antennas each include a main reflector 10, first feed means 12, and second feed means 14. The antennas also include at least one frequencyseelctive means cooperating with the feeds and reflectors, as will be explained below. In FIGURES l, 2 and 3, the two feeds in each antenna are located in the same region but are cross-polarized to each other, thereby achieving a substantial degree of independence. Thus in each of FIGURES 1, 2 and 3, the first feed means 12 is shown as a horizontally polarized horn feed for first waves of a first frequency f and second feed means 14 is shown as a vertically polarized dipole feed for second waves of a second frequency f feed means 14 being located close to the front of the horn feed 12. It will be appreciated that by applying design principles Well known in the art, these two cross-polarized feeds can be designed to operate substantially independently of each other.

One of the reflecting surfaces in each of the antennas in FIGURES 1, 2 and 3 incorporates a device called a frequency-selective twistreflector. Such devices have the properties of reflecting waves of both the frequencies involved, the polarization of the waves of one of the frequencies being changed to an orthogonal polarization (i.e., twisted by substantially in this case) while the polarization of the reflected wave of the other frequency is substantially unaffected. The term frequency-selective twistreflector refers to the combination of a frequency-selective means (16 in FIGURE 1) and a structural supporting device (the main reflector 10 in FIG- URE 1). Refer to the description of FIGURE 7 for a full explanation of such means, which may take the form of a conductive grating embedded in a dielectric skinflected by the frequency-selective means 16 with a 90 change in polarization. The resulting vertically polarized wave is focused toward dipole feed 14. Thus, the frequency-selective means cooperates with main reflector 10 and feeds 12 and 14 and distinguishes between waves on the basis of frequency so as to couple waves of the first frequency substantially only to feed 12 and waves of the second frequency substantially only to feed 14. A detailed explanation of focusing and feed operation is not required since once the basic idea is described, those skilled in the art will readily understand the operationof the antenna with the exception of the operation of the frequency-selective twistreflector which is explained in greater detail below with reference to FIGURE 7.

In FIGURE 2 there is shown a Cassegrain antenna including the main reflector 10, horizontally polarized horn feed 12, vertically polarized dipole feed 14 and subreflector 18. This antenna functions in the manner of known types of Cassegrain antennas except for the addition of a frequency-selective twistreflector in the form of a frequency-selective means 16 attached to, and supported by subreflector 18. Frequency-selective means 16 is similar to frequency-selective means 16 in FIGURE 1. This arrangement allows a Cassegrain antenna to operate with two waves of the same polarization but different frequencies, while providing isolation between the feeds which are cross-polarized. It will be understood that the drawings are not to scale and that actual Cassegrain antennas are designed with relatively small subreflectors in order to minimize aperture blocking.

In FIGURE 3 there is shown a Cassegrain antenna having a frequency-selective twistreflector in a form of frequency-selective means 16 attached to, and supported by, main reflector 10 (as in FIGURE 1). This antenna also includes a subreflector 22 and a transreflector 24. The subreflector 22 is of the type normally used in Cassegrain antennas and in the present case it would be used for the higher of the two frequencies used with the present antenna. The transreflector 24, is a device which substantially totally reflects waves of one polarization while appearing substantially invisible to waves of an orthogonal polarization which are transmitted substantially unaffected. This subreflector which may be constructed as described in applicants application Serial No. 173,501, filed February 15, 1962, and entitled Double-Reflector, Double-Feed Antenna for Crossed Polarizations and Polarization Changing Devices Useful Therein (see FIG. 5 therein) is effective to focus the vertically polarized waves of f frequency toward the vertically polarized dipole 14 while appearing invisible to the horizontally polarized waves of f frequency, which are then focused toward horizontally polarized horn feed 12 by subreflector 22. In this arrangement two different subreflector sizes are shown. The small solid subreflector would be permitted with the higher frequency because the feed can be highly directional. With the lower frequency a less directional feed would be likely and a larger subreflector would be required. As shown, the larger subreflector does not contribute to aperture blocking.

Antennas of FIGURES 4, 5, and 6 In FIGURES 4, 5, and 6 are shown three different types of Cassegrain antennas which include two feeds and which operate with free-space waves of two frequencies of the same polarization. These antennas each include a main reflector 10, two feed means, one or more subreflectors and at least one frequency-selective means as will be explained in more detail below. In FIGURES 1, 2 and 3, the two feeds in each antenna are spaced from each other as well as being cross-polarized to each other thereby allowing complete independence of design. Thus, each of these antennas includes a first feed means shown as a horizontally polarized horn feed for first waves of a first frequency f and second feed means shown as vertically polarized horn feed for second waves of a second feed frequency f Each arrangement is such that waves of one frequency are not focused toward the feed of the other frequency, as will be brought out below.

The antennas in FIGURES 4 and 5 each incorporate a device called a frequency-selective twistreflector whose properties have been discussed above. The antenna of FIGURE 6 includes frequency-selective means in a form of a frequency-selective transreflector. This device has the property of reflecting waves of one of the frequencies used with the antenna while appearing substantially invisible to the other frequency, and will be described in greater detail below with reference to FIGURE 8.

In FIGURE 4 there is shown a Cassegrain antenna having one feed 26 offset from the other feed 12 and having a cocked or tilted subreflector 24 (similar to that used in the FIGURE 3 antenna) and, in addition, a subreflector 22 which is of the type normally used in Cassegrain antennas. This antenna also includes a frequencyselective twistreflector in the form of frequency-selective means 16 attached to, and supported by, main reflector 10. In operation, the horizontally polarized feed 12 cooperates with the main reflector 10 and the subreflector 22 in the manner of a usual Cassegrain antenna with regard to horizontally polarized free-space waves of frequency f Horizontally polarized free-space waves of f undergo a change in polarization in reflection from the frequency-selective twistreflector, and cocked subreflector 24 substantially completely reflects the resulting vertically polarized waves of frequency 1; and acts to couple these waves to and from vertically polarized feed 26. Cocked subreflector 24 is substantially invisible to horizontally polarized waves and so does not affect the passage of these waves to or from subreflector 22.

In FIGURE 5 there is shown a Cassegrain antenna having a feed at each of the two focal points of the antenna system and having a subreflector 24 (similar to that used in the FIGURE 3 antenna) and a main reflector 10 with a frequency-selective means 16 attached to it. In operation, horizontally polarized feed 30 cooperates with the twist-reflector (which behaves substantially as a simple reflecting surface for waves associated with feed 30) substantially independently of subreflector 24 (which is invisible to horizontally polarized waves). Horizontally polarized waves of a frequency 72, are twisted substantially in reflection, by the frequency-selective twistreflector. The resulting vertically polarized waves of frequency f are coupled to vertically polarized feed 28 by subreflector 24. The antenna of FIGURE 5 not only has separately located feeds, but is symmetrical and there is no aperture blocking by the subreflector for either frequency-a very important consideration.

In FIGURE 6 there is shown a Cassegrain antenna having a feed at each of the two focal points of the system and having a subreflector 32 in the form of a frequencyselective transreflector (see FIGURE 8). This subreflector 32 reflects horizontally polarized waves of a frequency f but appears substantially invisible to horizontally polarized waves of frequency f Use of such a frequency-selective transreflector allows the antenna FIG- URE 6 to utilize two feeds of the same polarization while still achieving complete isolation and therefore allows complete independence of the design of the two feeds.

F requency-selective twistreflector of FIGURE 7 Referring now to FIG. 7 there is illustrated one form of frequency-selective twistreflector in accordance with the invention, which is useful in certain of the antennas described above. The twistreflector of FIG. 7 comprises a reflecting surface shown as surface 41, support means attached to the surface shown as means 46 and array means shown as dielectric skin 44 with grating 42 embedded therein.

In greater detail, the twistreflector as illustrated is made up of two principal structures: a member 40 which may be a portion of main reflector such as 10 in FIG. 1 or a portion of a subrefiector such as 18 in FIG. 2; and frequency-selective means 16 which comprises everything in FIG. 7 except member 40 which supports means 16. Frequency-selective means 16 as illustrated, comprises a grating 42 embedded in dielectric skin 44, support means 46 and a reflecting surface 41. For purposes of generality in describing and claiming the invention it will be helpful to consider the reflecting surface 41 as a thin conductive film attached to support means 46. As a practical matter, when the frequency-selective means 16 are attached to a main-reflector for example, as shown in FIG. 1, the surface of the reflector provides the reflecting surface and no thin conductive film is actually required. However, whenever frequency-selective means 16 are mentioned in this specification, means 16 must be considered to include a reflecting surface shown as 41 in FIG. 7a. The grating 42 is illustrated as a parallel arrangement of metallic wires embedded in a dielectric skin 44 which may be fiberglass, for example. The support means 46 may take the form of a foam or honeycomb material or may comprise many small structural support members utilized along the same principles as supporting beads commonly used in certain coaxial transmission lines. The overall effective dielectric constant of means 46 is desirably low even though the make-up may be a combination of high dielectric constant material and a lot of air as in bead supported transmission lines.

The particular design shown in FIG. 7 is especially suited for frequency ratios of approximately two. In operation, a wave at the low frequency, incident on the twistreflector of FIG. 7a from the right and having a linear polarization as indicated by arrow 45 in FIG. 7b, will be reflected with a linear polarization as shown by arrow 47. A wave at the higher frequency, incident in the same manner and having a linear polarization as indicated by arrow 45, will be relected without change in polarization (i.e., with a linear polarization as indicated by arrow 45). These results are due to the frequency-selective properties of the combination of the array means (grating 42 embedded in dielectric skin 44) in spaced relation to the conducting surface (41 in FIG. 7a).

Below are given design formulas and dimensions of a device actually constructed. In view of these formulas, many variations of frequency-selective twistreflectors in accordance with the invention will be apparent to those skilled in the art. Thus it is clear that other dimensions could be used, that more than one array or skin could be employed to achieve particularly frequency-selective characteristics, and that the arrays need not be parallel wires embedded in fiberglass as illustrated but any physical arrangement for producing the desired result can be substituted by those skilled in the art using known techniques. Inherent in these formulas are the basic concepts of the invention wherein:

A. the array means cooperate with the spaced reflecting surface so that incident waves of a first frequency are reflected substantially independently of the direction of polarization of these waves; but,

B. the array means reflect a portion of a first polarization component of incident waves of a second frequency and a predetermined polarization and transmit any remaining portion of that first polarization component together with substantially all portions of the orthogonal compo nent with the result that these waves of the second frequency are reflected with a change in polarization as a result of the recombination of the reflected portion and the transmitted portions. The phase of the reflected portion and the transmitted portions having been shifted by different amounts as a result of the presence of the array means.

As mentioned, the particular design shown in FIG. 7 is especially suited for frequency ratios of about two. In this design, Formulas 1, 2 and 3 below permit the dimensions L (separation of grating 42 from reflecting surface 6 41) and T (thickness of skin 44) to be calculated from given conditions which include A (wavelength at the low frequency) A (wavelength at the high frequency), cos 0 (average cosine of the angles of incidence of waves on the surface of skin 44) and K (dielectric constant of skin L in Formula 2 can be considered merely an intermediate operator allowing easier computation but having no easily explainable physical definition. Formula 4 merely relates S (separation of elements of grating 42) and D (grating elements diameter).

The resulting frequency-selective twistreflector is a wideband, wide-angle twistreflector at the low frequency and a moderately wideband wide-angle ordinary reflector at the high frequency. While physical designs which are superficially similar have been used before (see for example applicants applications Serial No. 80,961, filed January 5, 1961, now Patent No. 3,161,879, and entitled Twistreflector and Serial No. 94,513, filed March 9, 1961, now Patent No. 3,235,870, entitled Double-Reflector Antenna With Polarization-Changing Subreflector) it is not believed that frequency-selective properties such as here obtained have been achieved before. Thus the frequency-selective properties are here obtained by making use of a particular thickness (T) of dielectric skin 44- (Which supports grating 42) whereas earlier arrangements use a thin skin only for support without actual concern over its thickness aside from trying to keep it fairly thin.

Following are given final dimensions in wavelengths of an actual frequency-selective twistreflector which was constructed and tested and found to perform very well. These dimensions approximate those obtained from the formulas, but there are small differences occurring as a result of experimental modifications and practical deviations from the assumed construction.

IL M 1.84 .28

0 avg.=20 =.032

K s 4.0 g .186

Frequency-selective transreflector of FIGURE 8 Referring now to FIG. 8, there is shown one form of a frequency-selective transreflector in accordance with the invention. This transreflector comprises first array means shown as a grid 48 embedded in a dielectric skin 50, second array means shown as grid 52 embedded in skin 54, and spacing means shown as low-dielectric-constant material 56. The grids 48 and 52 are illustrated as parallel conductive wires and are embedded in dielectric skins 50 and 54, which may be fiberglass, for example. Spacing means 56 can take any of the forms discussed in connection with means 46 in FIG. 7a. The transreflector illustrated in FIG. 8 is especially suited for frequency ratios of about two. At the high frequency, the two grids 48 and 52 are spaced to form a half-wave resonator which transmits an incident wave polarized parallel to the straight parallel wires as indicated by arrow 60. At the low frequency these two grids 48 and 52 are approximately one-fourth of a wavelength apart and reflections from them reinforce, so that an incident wave polarized parallel to the wires is almost completely reflected. An additional factor contributing to this electrical performance is the decreasing effect of each wire grid as frequency increases. Once the principles of operation of a frequency-selective twistreflector are appreciated, many variations in actual physical construction will occur to those skilled in the art. Thus, it is clear that the spacing of the arrays may be any odd multiple of one-fourth wavelength at the lower frequency and any multiple of one-half wavelength at the higher frequency and is not limited to the particular dimensions shown and described. There may also be more than two arrays, in order to achieve particular frequency-selective characteristics. Also the arrays need not be parallel to Wires embedded in fiberglass as illustrated but any physical arrangement for producing the desired result can be substituted.

This frequency-selective transreflector has essentially the same structure as well-known devices used to provide a transreflector which reflects one polarization while appearing substantially invisible to an orthogonal polarization. However, as described above, the dimensions of the present structures achieve the desired frequency-selective feature. It should be pointed out that the feature of polarization sensitivity inherent in the frequency-selective transreflector is not required in the antenna of FIGURE 6, but is inherent in the particular construction used.

It will be appreciated that while the conductors in a grating are described as being parallel to each other, the grating will often be in the form of a concave or convex surface so that individual conductors actually have differ ent curvatures and are not strictly, completely parallel in all planes. The conductors are, however, usually parallel to each other when viewed from a distant position directly in front of the antenna. The term parallel" is relied upon because it is considered to aid in visualizing the physical configuration without complicating the description and claims to the degree that an absolute quantitative description would. This usage should be kept in mind in reading the present specification.

Circular polarization In all of the illustrated antennas, with the exception of FIGURE 6, the antenna can be made to operate with circular-polarization merely by changing the frequencyselective twistreflector to a frequency-selective circularpolarizing reflector. The design of frequency-selective circular-polarizing reflectors is straight forward in view of the present invention and the detailed description of polarization changing devices in applicants application Serial No. 173,501, filed February 15, 1962, entitled Double-Reflector Double-Feed Antenna for Cross Polarizations and Polarization Changing Devices Useful Therein.

In the antenna of FIGURE 6, operation with circularpolarization can be achieved by changing the simple main reflector to a nonfrequency-selective circular-polarizing reflector of the type described in the just-mentioned application. In connection with the frequency-selective twistreflector such as that illustrated in FIGURE 7, it should be appreciated that in some applications operation with circular polarization may be desired. In this case the selective twistreflector would reverse the sense of rotation of the circular polarization at the first frequency, just as does an ordinary reflector. At the second frequency where the twisting feature becomes operative, the selective twistreflector would not reverse the sense of rotation of the circular polarization; such action is opposite to that of an ordinary reflector. To avoid undue complication in the description and claims, that action which corresponds to an ordinary reflector will be regarded as being no change in polarization, and that action which does not correspond to an ordinary reflector will be regarded as a change of polarization.

While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed to cover all such changes and modifications as fall within the true spirit and scope of the invention. For example, it will be apparent that in the antenna of FIGURE 6 as shown, aperture blocking can be eliminated by adding a nonfrequency-selective twistreflector to the reflecting surface of the main reflector.

What is claimed is:

1. A double-feed antenna operating with two free-space polarized waves of the same polarization but of different frequencies comprising:

a main reflector;

first feed means for first polarized waves of a first frequency;

second feed means for second polarized waves of a second frequency;

and frequency-selective means cooperating with said main reflector and said feeds for changing the polarization of waves of said second frequency.

2. A double-feed double-reflector antenna operating with two free-space polarized waves of the same polarization but of different frequencies comprising:

a main reflector;

a subreflector;

first feed means for first polarized waves of a first frequency;

second feed means for second polarized waves of a second frequency;

and frequency-selective means cooperating with said main reflector, said subreflector and said feeds for changing the polarization of waves of said second frequency.

3. A double-feed double-reflector antenna operating with two free-space waves of a first polarization but of different frequencies, comprising:

a main reflector;

a subreflector;

first feed means for first waves of a first frequency and said first polarization;

second feed means for second waves of a second he quency and a second polarization;

and frequency-selective means cooperating with said main reflector, said subreflector and said feeds for changing the polarization of said second waves to said first polarization during transmission and for changing the polarization of waves of said second frequency to said second polarization during reception.

4. A double-feed antenna operating with two free-space waves of a first polarization but of different frequencies, comprising:

a main reflector;

first feed means for first waves of a first frequency and said first polarization;

second feed means for second waves of a second frequency and a second polarization;

and frequency-selective means coupled to said main reflector for changing the polarization of said second waves to said first polarization during transmission and for changing the polarization of waves of said second frequency to said second polarization during reception.

5. A double-feed double-reflector antenna operating with two free-space waves of a first polarization but of different frequencies comprising:

a main reflector;

a subreflector;

first feed means for first waves of a first frequency and said first polarization;

second feed means for second waves of a second frequency and a second polarization;

and frequency-selective means coupled to said subreflector for changing the polarization of said second waves to said first polarization during transmission and for changing the polarization of waves of said second frequency to said second polarization during reception.

6. A double-feed double-reflector antenna operating with two free-space waves of a first polarization but of different frequencies, comprising:

a main reflector;

a subreflector;

first feed means for first waves of a first frequency and said first polarization;

second feed means for second waves of a second frequency and a second polarization;

and frequency-selective means coupled to said main reflector for changing the polarization of said second waves to said first polarization during transmission and for changing the polarization of waves of said second frequency to said second polarization during reception.

7. A double-feed triple-reflector antenna operating with two free-space waves of a first polarization but of different frequencies, comprising:

a main reflector;

a first subreflector;

first feed means cooperating with said first subreflector for first waves of a first frequency and said first polarization; second feed means for second waves of a second frequency and a second polarization;

frequency-selective means coupled to said main reflector for changing the polarization of said second waves to said first polarization during transmission and for changing the polarization of waves of said second frequency to said second polarization during reception;

and a second subreflector spaced between said main reflector and said subreflector for transmitting waves of said first polarization and reflecting waves of said second polarization.

8. A double-feed triple-reflector antenna operating with two free-space waves of a first polarization but of different frequencies comprising:

a main reflector;

Ia first subreflector;

first feed means cooperating with said first subreflector for first waves of a first frequency and said first polarization;

second said means spaced from said first feed means for second waves of a second frequency and a second polarization;

frequency-selective means coupled to said main reflector for changing the polarization of said second waves to said first polarization during transmission and for changing the polarization of waves of said second frequency to said second polarization during reception;

and a second subreflector spaced between said main reflector and said subreflector and cocked so as to [focus waves into said second feed, for transmitting waves of a first polarization and reflecting waves of said second polarization.

9. A double-feed double-reflector antenna operating with two free-space waves of a first polarization but of different frequencies, comprising:

a main reflector;

first feed means cooperating with said main reflector for first waves of a first frequency and said first polarization;

second feed means spaced from said first feed means for second waves of a second frequency and a second polarization;

subreflector means cooperating with said main reflector and said second feed for transmitting said first waves and reflecting said second waves;

and frequency-selective means coupled to said main reflector for changing the polarization of said second waves to said first polarization during transmission and for changing the polarization of waves of said second frequency to said second polarization during reception.

10. A double-feed double-reflector antenna operating with two free-space waves of a first polarization but of dilferent frequencies, comprising:

a main reflector;

first feed means cooperating with said main reflector for first waves of a first frequency and said first polarization;

second feed means spaced from said first feed means for second waves of a second frequency and a second polarization;

polarization sensitive subreflector means cooperating with said main reflector and said second feed for transmitting waves of said first polarization and reflecting waves of said second polarization;

and frequency-selective means coupled to said main reflector for changing the polarization of said second waves to said first polarization during transmission and for changing the polarization of waves of said second frequency to said second polarization during reception.

11. An antenna in accordance with claim 5 wherein the frequency-selective means cooperate with the subreflector to form a twistreflector comprising the reflecting surface of the subreflector, a grating of spaced parallel conductive members embedded in a skin of dielectric material and a supporting dielectric material supporting said grating and skin in spaced parallel relation to said reflecting surface.

12. An antenna in accordance with claim 6 wherein the frequency-selective means cooperate with the main reflector to form a twistreflector comprising the reflecting surface of the main reflector, a grating of spaced parallel conductive members embedded in a skin of dielectric material and a supporting dielectric material supporting said grating and skin in spaced parallel relation to said reflecting surface.

13. An antenna in accordance with claim 7 wherein the frequency-selective means cooperate with the main reflector to form a twistreflector comprising the reflecting surface of the main reflector, a grating of spaced parallel conductive members embedded in a skin of dielectric material and a supporting dielectric material supporting said grating and skin in spaced parallel relation to said reflecting surface.

14. An antenna in accordance with claim 9 wherein the frequency-selective means cooperate with the main reflector to form a twistreflector comprising the reflecting surface of the main reflector, a grating of spaced parallel conductive members embedded in a skin of dielectric material and a supporting dielectric material supporting said grating and skin in spaced parallel relation to said reflecting surface.

15. A frequency-selective transreflector, for transmitting waves of a first frequency and polarization and reflecting waves of a second lower frequency and the same polarization, comprising:

a plurality of array means for reflecting portions of incident waves of both said frequencies and transmitting the remaining portions of said waves;

and spacing means attached to said array means for supporting them in spaced parallel relation with a spacing such that there is a separation between array means which approximates a multiple of one-half of the effective wavelength at the first frequency and an odd multiple of one-quarter of the effective wavelength at the second lower frequency;

whereby, the complete transreflector provides substantially the function of a half-wave resonator for Waves of the first frequency and at the second lower frequency the reflections from the array means reinforce so that waves at the lower frequency are substantially completely reflected.

16. A frequency-selective transreflector for appearing substantially invisible for Waves of a first polarization and a first frequency and for reflecting waves of said first polarization but of a second frequency which is equal to approximately half of said first frequency comprising:

two gratings of spaced parallel conductive members;

a dielectric skin surrounding and supporting each of said gratings;

and a dielectric structure supporting these two gratings and skins parallel to each other with the two gratings spaced from each other so as to form a halfwave resonator which transmits incident waves of the first frequency which are polarized parallel to the wires and so that at said second frequency the two gratings are approximately a quarter-wave apart and reflections from the two gratings reinforce causing an incident wave of said second frequency which is polarized parallel to the wires to be almost completely reflected.

17. A double-feed double-reflector antenna operating with two free-space waves of the same polarization but of different frequencies comprising:

a main reflector; first feed means for first Waves of a first frequency and said same polarization;

second feed means for second waves of a second frequency and said same polarization;

and frequency-selective su'breflector means cooperating with said main reflector and said feeds for transmitting waves of said first frequency and reflecting waves of sai-d second frequency, said subreflector means comprising two gratings of spaced parallel conductive members each grating embedded in a skin of dielectric material and a supporting dielectric material supporting the two skins and gratings in spaced parallel relation to each other.

References Cited by the Examiner UNITED STATES PATENTS 2,530,580 11/1950 Lindenblad 343-755 X 2,736,895 2/ 1956 Cochrane 343-755 X 2,790,169 3/1957 Sichak 343-756 2,870,444 1/ 1959 Broussaud 343-909 2,930,039 3/1960 Ruze 343-756 2,930,040 3/ 1960 Weil 343-756 3,025,515 3/1962 Fairbanks 343-840 3,161,879 12/1964 Hannah et a1. 343-756 X 3,165,749 1/1965 Cushner 343-909 X 3,195,137 7/1965 Jakes 343-756 FOREIGN PATENTS 562,602 9/ 1958 Canada.

HERMAN KARL SAALBACH, Primary Examiner.

E. LIEBERMAN, Assistant Examiner. 

1. A DOUBLE-FEED ANTENNA OPERATING WITH TWO FREE-SPACE POLARIZED WAVES OF THE SAME POLARIZATION BUT OF DIFFERENT FREQUENCIES COMPRISING: A MAIN REFLECTOR; FIRST MEANS FOR FIRST POLARIZED WAVES OF A FIRST FREQUENCY; SECOND FEED MEANS FOR SECOND POLARIZED WAVES OF A SECOND FREQUENCY; AND FREQUENCY-SELECTED MEANS COOPERATING WITH SAID MAIN REFLECTOR AND SAID FEEDS FOR CHANGING THE POLARIZATION OF WAVES OF SAID SECOND FREQUENCY. 